Releasing biological control agents

Timing of releases

Synchronicity

A new control agent can only establish if the target life stage of the host is
present when the agent is released. Accurate timing of releases is therefore
critical to establishment success. Even though univoltine or bivoltine pest
species may be present year round, the life stages that are susceptible to the
control agent may be present for a relatively short period. Releases must
coincide with this window. In weed control, the susceptible plant structure at
the right stage of development must be present when the agent is released. For
example, Hill et al. (2000) found that the eurytomid seed-feeding wasp only laid
eggs into the seeds of Acacia species for 3 weeks in late spring. If adult wasps
were released outside this period, no suitable seeds would be available for
oviposition. Similarly, Dymock (1987) found that adult ragwort seed fly,
Botanophila jacobeae began emerging in late October and early November, 4-6
weeks before ragwort began to flower. During this time they fed in order to
mature eggs. Release of new adults in early December, when flowers suitable for
oviposition first appear, might not allow sufficient time for reproductive
development before the resource became too mature for larval development.

Synchrony can be manipulated by seeding release sites with target pests of the
correct stage, by introducing plants to the release site that are susceptible to
the target pest out of season, or by judicious pruning to alter host plant
phenology.

Seasonality

Seasonality and synchronicity are closely linked concepts. The period of the
year during which agents can be released may be limited by the need to expose
the control agents to climatic requirements that condition insects for
successful diapause or hibernation. For weed control agents, it may be necessary
to release agents during a narrow window when host-plant quality is adequate for
successful development.

Hill (1982) found that the protein content of gorse foliage was generally low,
peaking for only a short period in the spring. The life history of most foliage
herbivores was keyed to this resource. Strategies for releasing foliage-feeding
biological control agents in New Zealand were designed with this in mind (e.g.
Hill et al. 1995).

Where an exotic agent is already established, or where native species are
active, newly-introduced agents might face competition for hosts. Where
possible, release multivoltine agents at a time of year when competition is at a
minimum. Similarly, predators and host disease can interfere by removing
susceptible stages of the pest, or even destroying parasitised individuals.
Where such interactions are known, time releases to minimise such interactions.

References

Dymock J.J. (1987).
Population changes of the seedfly, Pegohylemyia jacobaeae (Diptera: Anthomyiidae) introduced for biological control of ragwort.
New Zealand Journal of Zoology 14:337-342.

Hill R.L., Gordon A.J. and Neser S. (2000).
The potential role of Bruchophagus acaciae (Cameron) (Hymenoptera: Eurytomidae) in the integrated control of Acacia species in South Africa.
Pp. 919-929 In: Proceedings of the X International Symposium on Biological Control of Weeds, N.R. Spencer (Ed.) Montana State University, Bozeman, Montana, USA.